Hydrodynamic bearings have been proposed for supporting spindle assemblies within disk drives. These prior approaches have typically employed one type of hydrodynamic bearing known as a spiral groove bearing. The spiral groove journal bearing features a unitary shaft and a unitary sleeve, both being sized, formed and mounted for rotation relative to one another. Hydrodynamic action in the bearing is achieved by providing one of two facing surfaces of each journal bearing region with a suitable helical groove pattern for pumping bearing lubricant during relative rotation, while the other facing surface of the journal bearing region is typically very smooth.
Hydrodynamic bearing spindle systems are provided and used in contemporary disk drive spindles as self-contained bearing systems. A typical self-contained hydrodynamic spindle bearing system includes a shaft and a sleeve cooperatively defining a pair of spaced-apart radial grooved journal bearings and a pair of axial grooved thrust bearings, with sealing mechanisms located at the system boundaries with the ambient environment. Common sealing mechanisms include surface tension capillary seals, seals formed with barrier film coatings, seals formed with divergent geometries, seals formed with pumping grooves, and combinations of the foregoing mechanisms. The seals are needed to prevent loss of lubricating liquid. Loss of lubricant must be minimized in order to prevent lubricant starvation of the bearing leading to spindle failure. Of equal concern within hard disk drive closed environments, loss of lubricant into the head-disk environment may lead to contamination of the head-disk interface, and resultant head crashes and damage to the head and/or disk and resultant loss of the drive's usefulness.
The advantages and drawbacks of using pumping grooves for sealing are explained, for example, in U.S. Pat. No. 4,596,474 to Van Roemburg, entitled: "Bearing System Comprising Two Facing Hydrodynamic Bearings"; and in U.S. Pat. No. 4,798,480 to Van Beck, entitled: "Bearing System with Active Reservoir Between Two Axially Spaced Hydrodynamic Bearings". The disclosures of these two patents are incorporated herein by reference.
When surface tension capillary seals and/or barrier film seals are employed, it is necessary to prevent pressure build-up otherwise resulting from net groove pumping action which exceeds the sealing capability of the particular sealing mechanism. Otherwise, the pressure build-up will lead to a net flow of lubricant along an axial direction of relative rotation and unwanted lubricant migration out of the bearing system. Because the fluid pumping forces from the spiral or helical grooves are typically one or more orders of magnitude larger than the surface tension forces present at the capillary seal, a common practice has been to design self-contained hydrodynamic bearing systems with pressure balancing groove patterns. For example, pressure balancing within a single journal bearing may be achieved by using two helical groove patterns which cooperate to define a symmetrical herringbone pattern.
It has been previously discovered that closed-loop recirculation of lubricant between outer and inner capillary seals of two spaced-apart thrust bearings of a self-contained bearing system reduces at least static pressure otherwise tending to cause surface tension capillary seals to leak, see U.S. Pat. No. 5,112,142 to Titcomb et al., entitled "Hydrodynamic Bearing", the disclosure thereof being incorporated herein by reference.
A pronounced trend toward miniaturization is present with contemporary disk drive designs in both overall form factor, and in the disk spindle height dimension. Because of size compaction of disk drive spindles, the length of a typical herringbone helical groove bearing used in the disk spindle is approximately 3 mm (0.12"). A 0.03 mm (0.001") offset of the herringbone apex from a location of symmetry may be sufficiently large enough to create an unbalanced pumping force which results in lubricant leakage. Accordingly, extremely stringent tolerances are required during manufacturing in order to achieve a usable pair of pressure-balancing helical grooves forming the herringbone pattern. This high level of required precision in turn has heretofore required very high manufacturing costs and has resulted in unacceptably low yields within a mass production manufacturing environment.
While manufacturing processes are well known for forming the journal bearing regions of a shaft-sleeve hydrodynamic bearing assembly, at least two independent machining processes and set-ups are typically required. A first machining process, carried out on a precision numerically controlled lathe, results in formation of raised cylindrical walls on either the shaft, or the sleeve, of the bearing assembly, separated by an inner relieved cylindrical area forming a continuous reservoir for lubricant between the two journal bearings and by two axially outer lubricant reservoirs leading to the sealing regions at the ambient interface. Hydrodynamic axial thrust bearings may also be formed between the journal bearing outer lubricant reservoirs and the sealing regions.
After the shaft and/or sleeve has been machined on the lathe to define a raised cylindrical region to be grooved, the machined part is then subjected to a separate groove-forming process. For example, the part or article to be grooved may be installed on a separate grooving machine, such as one employing the precision ball coining or embossing technique described in U.S. Pat. No. 5,265,334 to Lucier, entitled: "Device for Manufacturing a Groove Bearing, and Method of Manufacturing a Groove Bearing by Means of the Device", the disclosure thereof being incorporated herein by reference. Precise groove patterns are said to be achieved in accordance with the methods of the '334 patent by simultaneous translation and rotation of the forming ball or balls within the forming machine relative to the cylindrical raised surface to be grooved. Other grooving techniques may be employed, such as selective patterning of etch resist and chemical wet etch, grinding under precise numerical control, ablation by selective positioning and activation of a laser, etc. Irrespective of the particular groove forming method, it is conventionally carried out as a step which is subsequent to and separate from the step of forming the raised cylindrical region.
In order to form a precise herringbone groove pattern on each journal bearing region following the lathing step, in accordance with the prior art approaches the grooving machine must be referenced precisely to a starting location, and ending location, of each raised cylindrical journal bearing region to receive groove-forming ball coining process. Such locating presupposes that each raised bearing region was turned to a precise cylindrical length after being referenced with respect to a precise fiducial datum plane by the lathe, and further presupposes that the ball-grooving machine is thereafter aligned with the same precise fiducial datum plane prior to forming the precise herringbone groove patterns into each raised journal region. The same considerations apply to forming grooves on raised journal portions of the shaft, as opposed to the sleeve, should this alternative arrangement be desired. Holding extremely tight tolerances during manufacturing is extremely difficult and costly, and constitutes a major drawback in adoption and use of hydrodynamic bearing systems within mass produced motors, such as spindle motors for miniature hard disk drives. Alternatively, the steps of measuring actual tolerances of each machined article, and adjusting the grooving machine to compensate for tolerances, slows production and introduces further possible sources of error into the grooving process.
Thus, a hitherto unsolved need has arisen for a manufacturing method for forming pumping grooves within a spiral groove hydrodynamic bearing system which overcomes the limitations, costs and drawbacks otherwise associated with prior groove forming methods.